1. Relation to Other Disclosures
This disclosure is related to U.S. patent application No. 103,459, now U.S. Pat. No. 4,809,237, entitled Method for Monitoring the Goodness of the Cement Bond to a Borehole Casing, which is filed concurrently with this application.
2. Field of the Invention
This invention relates to acoustic borehole logging tools for use in investigating the properties of the borehole sidewall material, such as the goodness of the cement bond to the borehole casing. Such tools may be found in class 367.
3. Discussion of the Prior Art
Acoustic logging tools for measuring properties of the sidewall material of a borehole are well known. Basically, such tools measure the travel time of an acoustic pulse over a known distance through the sidewall material. From those data, the acoustic-wave propagation velocity is calculated. For some applications, the character of the seismic or acoustic waveform is also of interest. Criteria of interest are amplitude and frequency.
Acoustic waves propagate through elastic media in various modes. Of interest are compressional or P-waves, wherein particle motion is in the direction of wave travel, and transverse or S-waves wherein particle motion is perpendicular to the direction of wave travel. Other types of waves propagate in the borehole such as pseudo-Raleigh wave, Stonley wave, tube waves, and compressional waves directly through the mud column. Those other waves are interesting but often are more of a nuisance than otherwise.
S-waves are distinguished from P-waves chiefly by their velocity. The P-wave velocity is determined by the elastic constants and the density of the medium through which the P-waves propagate. The S-wave velocity is less than the P-wave velocity in the ratio ##EQU1## where .sigma. is Poisson's ratio which is about 0.25 for idealized elastic solids. The S-wave velocity is therefore 0.5773 V.sub.p or, for practical purposes, about half of the P-wave velocity. In fluid, .sigma.=0.5 and V.sub.p /V.sub.s =.infin.. Hence, S-waves cannot propagate through a fluid.
When a compressional or P-wave is generated in the borehole fluid, it may be refracted at the Snell's-law critical angle into the formation at the interface between the borehole fluid and the formation. At that point, both P-waves and S-waves are generated which propagate through the formation. The P-wave may be refracted back into the fluid at or beyond the so-called critical distance where it may be detected as a primary compressional wave. However, the S-wave cannot escape from the formation because the borehole fluid will not support S-wave propagation. What does happen, however, is that the S-waves propagate through the sidewall material, mechanically exciting corresponding compressional waves in the fluid, giving rise to converted-compressional waves. The arrival time and the envelope of the converted compressional waves are said to be representative of certain S-wave characteristics.
For critical-angle refraction to occur, the sidewall material must be characterized by a propagation velocity that is greater than that of the borehole fluid. In soft or sloughing formations, the S-wave propagtion velocity may be less than that of the fluid. S-waves from critical-angle refraction, therefore, are not present. It has been found that in such circumstances, S-waves can be generated by applying a normally-incident compressional pulse, sideways, directly against the sidewall material. The effect, termed direct excitation, may be likened to application of a hammer-blow to the sidewall.
Direct excitation, as taught herein, should be distinguishd from so-called point-force excitation. In the former case, the transducer dimensions are comparable to the wavelength of the acoustic radiation. In the case of point-force excitation, the transducer dimensions are negligible as compared with the wavelength of the radiated acoustic waves, perhaps more than one order of magnitude less than that wavelength.
In bore-hole logging, data provided by both P-waves and S-waves are of interest for diagnostic studies. Information so derived permits the determination of formation elastic constants, rock texture, rock fluid content, formation fracturing and the goodness of cement bonding to the borehole casing. For some studies, however, it has been found that S-wave information is superior to P-wave information.
S-waves may be directly detected by suitable sensors that are mounted on pads that are pressed into direct contact with the borehole sidewall such as taught by U.S. Pat. No. 3,376,950. The disadvantage of such tools is that the sensors and the pads are severely abraded as the tool is moved along the borehole. Further, that motion generates a good bit of "road noise". Direct-contacting tools have not been found to be very satisfactory.
A number of logging tools have been proposed wherein the tool is suspended directly in the borehole fluid but carefully separated from physical contact with the borehole wall, usually by centralizers or bumpers. Those disclosures purport to detect transverse or S-waves. For reasons explained earlier, those tools detect converted-compressional waves, but not, strictly speaking, transverse waves.
Typical prior-art tools are exemplified by Caldwell, U.S. Pat. No. 3,333,238, wherein he employs symmetrical cylindrical transducers to generate and presumably receive transverse waves. He is interested in the S-wave amplitude. To avoid interference with other acoustic arrivals, he provides electronic delay-gating to preferentially receive the desired signals.
White, U.S. Pat. No. 3,330,375, teaches a wavelength tuning technique to identify the various waveforms on the basis of propagation velocity. He employs an array of receivers to provide a spatial interference filter that discriminates in favor of selected wavelengths.
Pickett, et al, in U.S. Pat. No. 3,276,533, employs a piezoelectric receiver and several magnetostrictive transmitters to selectively record slow acoustic arrivals that have traveled over various distances separating the receiver and transmitter. He is interested in providing character logs. Asymmetrical insonification is employed.
Ingram, U.S. Pat. No. 4,131,875, seeks preferentially to enhance later acoustic-wave arrivals relative to the early arrivals, the wavelengths of the later arrivals being much greater than the borehole diameter. The early and late arrivals are frequency-separable. A symmetrical transducer array consisting of a transmitter and several receivers is taught.
Zamanek, U.S. Pat. No. 4,516,228, teaches a logging tool for detecting both P-waves and S-waves. A symmetrical transmitter applies point-force pulses to the sidewall material. A dual-crystal piezoelectric receiver is provided. Electronic circuitry provides gating pulses wherein the signals of the dual-crystal receiver are subtracted such that the difference signal is representative of P-wave arrivals. After a suitable gated delay, the receiver operates in the asymmetrical mode to add the signals received by the crystal receiver to produce a sum signal representative of S-waves.
As mentioned above, for certain diagnostic measurements, use of S-waves is preferable to use of P-waves. For example, in cement-bond studies, it is known that for no cement bond or for a poor cement bond between formation and borehole casing, a strong compressional wave travels through the steel casing at a casing velocity of 17,000 feet per second (fps). If a good bond exists, the casing wave is strongly attenuated and the compressional formation-wave appears having a velocity on the order of 8-12,000 fps.
We have found that P-wave analysis of cement bonding is over-sensitive to the presence of a micro-annulus around the casing but is insensitive to the presence of small vertical channeling. A micro-annulus is defined as a slight separation between cement and casing of a few thousandths of an inch, say less than 0.015 inch. A micro-annulus is not considered to be a problem in respect to well completion insofar as cement-to-casing bonding is concerned. Yet the results of a P-wave study might indicate no cement bond at all, as indicated by a strong casing wave arrival, with the possible recommendation of an expensive and unnecessary cement squeeze job. We have discovered that transverse or S-wave studies are not significantly sensitive to the presence of a micro-annulus.
Another problem with P-wave studies arises when the logging tool is not exactly centered in the borehole. A deviation of as little as 1/16 of an inch has been found to produce unreliable test results. That problem is particularly true when using symmetrically-radiated acoustic energy as for cylindrical transducers. We have found that use of converted-compressional waves in conjunction with asymmetrically-configured sensors is substantially insensitive to so-called ex-centering.
Thus, we have discovered that the compressional-wave amplitude through the casing does not change very much even in the presence of significant channeling or micro-annuli. On the other hand, the relative converted-compressional wave amplitude will vary significantly in response to the goodness of the cement-to-casing bond, ignoring, however, micro-annuli, the amplitude being larger where the cement bond is poorer. Further, any variation of the relative converted-compressional wave amplitude is between transducer sets at different azimuths will indicate channeling or some other anomaly.